It is well known that tobacco smoke contains more than three thousand constituents in the form of liquid-solid or vapor. Cigarette filters commonly used now are made of cellulose acetate, cotton, rayon, or paper. Among these filter materials, over 90% of the filter cigarettes made in the U.S. and a majority of filter cigarettes made in the world use a single-segment cellulose acetate filter. The performance of these filters in terms of pressure drop generation and tar filtration efficiencies are somewhat limited because of certain requirements for cigarette filter. For example, at a filter length of between 15 mm and 30 mm, filter pressure drop is typically between 30 mm and 120 mm, and filtration efficiency is typically between 25% and 50%. Therefore, to make medium or low tar cigarettes, which requires higher than 50% filtration efficiency, ventilation of the filter tips is commonly used, which adds extra cost for the cigarette manufacturers.
The prior known fibrous filters are capable of removing varying percentages of tar and nicotine from cigarette smoke depending on the amount of fibrous material compacted into them, their length, their circumference, their resistance to draw, the surface characteristics of the fiber, and other factors. These filters, however, show substantially no reverse selectivity for the filtration of nicotine from cigarette smoke, particularly when the tobacco is the conventional type used in cigarettes produced in the United States.
U.S. Pat. Nos. 3,327,718, and 3,340,879 describe cigarette filters consisting of fibrous filter material treated with zeolite and poly(alkyleneimine), respectively, for removing acidic components in the smoke. In these U.S. patents, reverse selective removal of nicotine was not disclosed but it is probable that the removal of acidic components in the smoke would change the pH of the smoke, thus supressing the filtration of nicotine in the smoke. No. U.S. patent is known which specifically discloses suppressing the filtration of nicotine in cigarette smoke. Another U.S. Pat. No. 3,428,056 describes a filter material incorporated with a polyolefin powder on which inorganic water soluble salts are coated especially the salts such as sodium carbonate, sodium phosphite, potassium carbonate and potassium phosphite. These filter materials were used to improve particulate matter removal and to selectively remove hydrogen cyanide from tobacco smoke. UK patent 2,189,127 discloses polyolefin-containing or cellulose acetate filter substrate treated with sodium bicarbonate, sodium carbonate, potassium permanganate, manganese dioxide, dissolved or dispersed in a nonionic spin solution. Such filter substrates were used to remove undesirable components such as hydrogen cyanide. The present invention relates to filter additives in a certain form which will greatly enhance the filtration efficiency of tobacco smoke filters without increasing the filter pressure drop substantially.